One of the more intriguing v$event_histogram applications is gaining an undocumented understanding of Oracle's interworkings. For example, the free buffer wait event (which will be discussed in the next chapter) is documented to cause a 10 ms wait, period. However, if the event histogram view is to be trusted, then why have DBAs seen 15% of the wait times greater than 16 ms? (I have seen this many times.) This indicates there are other things occurring inside Oracle, which we as DBAs are not aware of, forcing us to look a little deeper into Oracle's inner workings.4

It was a long and painful struggle. When the wait interface was first available, no one knew about it, so convincing people it was valuable was in direct opposition to the established Oracle performance analysis community. Historically, going against a scientific establishment can mean you are publicly discredited and potentially searching for a new job. But after many battles and unfortunate personal attacks, the wait interface prevailed. However, the wait event community has kept pushing so hard that, in some cases, they have misrepresented the true performance situation, bringing into question some performance analyses based on wait event information. This section highlights three of the most common wait event myths. This is important, because as performance firefighters, we need to be aware of the wait interface limitations so we don't misdiagnose.

Decreasing wait time usually does improve performance, but not always. First, a user does not feel only wait time, but wait time plus service time. Second, through cyclical performance optimization cycles, our goal is to reduce response time-again, not wait time or service time, but their combination.